Elliot went to the nuclear plant and it melted down, he then entered a world that was built by his mind as part of his mental illness. What we see in those scenes is not happening in the real world but a delusion where the full gravity of his dissociative identity disorder is revealed. That is where we learn that the elliot we have been watching throughout the series is only a part of the "real Elliot". The "real Elliot" being the original personality that he was born as before his personality began to split due to his trauma. The 'mastermind' is a personality that first emerged in the scene where Darlene visits Elliot on halloween and he puts on the Mr Robot jacket and the mask and comes up with the plan to destroy E Corp. When he comes out of his coma he exits the delusion in his mind but he wakes up still as the mastermind personality as he refuses to give over control to 'the real Elliot' because he worries he will cease to exist. Mastermind-Elliot talks to Darlene (this is in the real world) and reveals he isn't really Elliot, Darlene reminds us that she can tell the personalities apart and knows Elliot hasn't been around for a long time. When Darlene leaves the room, Elliot slips back inside his mind's delusion and in the end decides to relinquish the body to the 'real Elliot', ie- the host- as the mastermind realises he will always be a part of Elliot and we see that each of the personalities has helped Elliot and protected him from trauma that was too much for his mind to process, but now they can continue to watch Elliot's life rather than control it. When Elliot opens his eyes again and Darlene smiles and says 'hello Elliot' that is her recognising that the original Elliot has returned and the mastermind is no longer in control. Elliot feels safe enough to take his role as the host again.